Temperature-resistant reflective coating for quartz envelope



Oct. 27, 1970 B. K OP ELIMAN ET AL 3,533I9Z TEMPERATURE-RESISTANTREFLECTIVE COATING FOR QUARTZ ENVELOPE Filed Dec. 7, 1967 BERNARDKOPELMAN MARSHALL E. KULBERG INVENTORS BY m A TORNEY FIG.2

United States Patent "ice US. Cl. 313-113 Claims ABSTRACT OF THEDISCLOSURE A diffusely-reflecting coating on quartz is made of asilico-phosphate. It can be used on a tubular quartz envelope toincrease the effective size of an incandescent filament coil for opticalpurposes. The coating can be applied as a mixture of phosphoric andsilicic acids with some ammonium bifluoride added, and then heated to atemperature of about 1125 C. A porous and adhesive white-coating isformed on the glass. Heating to 1150 C. will fuse the mixture to thequartz as a clear coating and cause cracking due to unequal coeflicientsof expansion. The coating can also be used on the quartz envelope of anelectric discharge tube such as a high pressure mercury vapor lamp, andthe coating can be used for other purposes. The coating can be made thinfor diffusion or heavy for reflection.

This invention relates to a diffusing coating suitable for use on quartzand high silica glasses. In particular, it relates to a lamp in a quartzenvelope having such a coating over at least part of its surface.

Lamps having extended area planar type filaments, such as the so-calledC13 and C13D filaments, have found wide use in slide projector equipmentand in fixtures used in television and motion picture studios. Theseplanar filaments often called monoplane or biplane filaments, dependingon whether the multiplicity of parallel filament coils of which they arecomposed is arranged in a single plane or in two slightly-spacedparallel planes, are usually made in soft or hard glass envelopes.

While it is possible to use coil configurations in quartz lamps atwattages of 1000 w. or less, the larger wattages with their larger coilconfigurations require such large diameter quartz tubes as to beeconomically unattractive. It appears, however, that if a single coil issurrounded partially by a substantially opaque diffusely-reflectingcoating, then the coating itself behaves as a spread-out filament andsimulates the behavior of the C13 coil, due to reflection of light fromthe coating. Such a coating on a quartz tube will allow a single coil tobehave like a C13 coil and allow tungsten-halogen lamps to becompetitive with soft and hard glass incandescent lamps in existingfixtures.

The quartz envelope of the tungsten-halogen almp operates at very hightemperatures, often well above 600 C., and ordinary coatings areunsatisfactory, because they will either crack the quartz, because ofits much lower expansion coeificient, or they will burn off.

We have discovered, however, that a coating of a silicophosphate can beused on the quartz without such cracking or burn-off and with veryeffective results.

When phosphoric acid, silicic acid and ammonium bifluoride (NH,HF aremixed, applied to quartz and properly heated, a white reflective coatingresults, and seems to have a porous nature and firm adhesion to thequartz.

The application of the mixture to the enevlope is quite critical. Afterbeing applied to the quartz as a paste, it is heated slowly; at about300 C., the liquids having 3,536,946 Patented Oct. 27, 1970 mostlyevaporated, it has the appearance of a grey frozen slush. At about 1000C. it begins to sinter to a porous but cohesive mix, and shows thebeginning of adhesion to the quartz at about 1125 C. However, themixture will fuse to a clear glass coating at about 1150 C. and thequartz envelope will eventually crack. Accordingly, the finaltemperature of the coating during manufacture must be below 1150 C., andabout 1125 C. being effective, and the temperature during subsequentoperation must be less than 1150 C. The latter limitation is not adisadavntage, however, since the tungsten halogen lamp generallyoperates well below that temperature.

The silico-phosphate is the only coating we have found that can be madeto adhere to quartz throughout the thermal cycling and recycling normalto the operation of the lamp. It appears to do this by a combination ofrelatively low thermal expansion coeflicient, only two or three timesthat of quartz itself, and a porous structure which takes up anystresses involved.

In order to adhere satisfactorily to the quartz, the

, silico-phosphate should contain an addition such as ammoniumbifluoride, which appears to etch the quartz surface so that a betterbond is achieved between the quartz and the silico-phosphate. Withoutsuch an additive, the coating will flake ofli. Other fluorides can :beused.

When ammonium bifluoride is used, it breaks down on heating to becomeammonium fluoride and hydrofluoric acid, the latter apparently etchingthe glass at the same time the coating is sintered, thereby forming agood bond.

In order to prevent cracking, the coating must not be heated to thepoint of complete fusion, but should be heated only to sintering orsemi-fusion.

Other objects, features and advantages of the invention will be apparentfrom the following specification taken in connection with theaccompanying drawings in which:

FIG. 1 is a perspective view of a lamp according to the invention; and

FIG. 2 is a cross-sectional view, showing the filament coils.

In FIG. 1, the tubular quartz envelope 1 has the pressed seal 2 at oneend and the sealed exhaust tube 3 at the other end. The external lead-inwires 4, 5' extend into the press seal 1, where each is attached in theusual manner to the thin molybdenum ribbons 6, 7 and act also as supportwires for the coiled-coil filament 10 shown in FIG. 2, but obscured inFIG. 1 by the coating 11 on the outside of envelope 1.

The coiled-coil tungsten wire filament 10 is mounted axially in thetubular envelope 1, the longitudinal axis of the coil beingsubstantially on the longitudinal axis of the tubular envelope 1, thecoil 10 being supported by the lead-in wires 8, 9, which are joinedtogether a short distance above the seal 1 by the corrugated quartz bead12. An additional support wire 13 extends to hold the middle portion ofthe filament coil 10, as shown more fully in FIG. 2

The lead-in and support wire 8 extends into the exhaust tube 3 to aid incentering and supporting the filament coil 10. Support wire 14 iswrapped around the upwardly-extending end 15 of wire 8 and extends fromthere in a bight 16 having an upwardly-extending end 17, which engagesthe singly-coiled end of the coiledcoil filament 10. The lead-in wire 9extends upwardly and laterally, terminating in the downwardly-extendingportion 18 to which an end 19 of filament coil '10 is affixed.

The filament structure and mounting can be shown in a copendingapplication :Ser. No. 683,755 of L. S. Huston, Jr., filed Oct. 31, 1967,for an Incandescent Lamp and the bead 12 can be shown in a copendingapplication Ser. No. 681,519 filed on Nov. 8, 1967, now Pat. No.

3 3,466,489, by Emery G. Audesse and L. S. Huston, Jr., with small coilsaround the lead-in wire inside the bead 12, if desired. Each of theseapplications has a common assignee with the present application.

The coating 11 can be applied to the quartz as a mixture of 20 grams ofsyrupy phosphoric acid, grams of silicic acid having a bulk-density ofabout 6.0 grams per cubic centimeter, and 2 grams of ammoniumbifiuoride, results in a paste which can be applied in a layer. Thecoating is then slowly heated to 300 C., cooled, a second layer appliedand heated similarly and then a third. A bubbly white coating results.It is then heated to 1125 C. If heated for minutes, the coating remainsa bubbly white with excellent adhesion and good reflectance. If heatedfor A of an hour, the material sinteres to an advanced sintered state,'which could be called semi-fused. Adhesion is excellent and no crackingof the quartz occurs.

The resultant lamp should not be operated at a temperature of 1100 C. orabove, because at such temperatures the silico-phosphate coating willeventually change to a clear glaze and lose its reflecting power. Threelayers of silico-phosphate, each about 3 millimeters thick, form a veryeffective coating.

Varioius modifications will be apparent from the foregoing specificationwithout departing from the spirit and scope of the inevntion, which islimited only by the claims.

What we claim is:

1. An electric lamp comprising a tubular quartz envelope, a light sourceinside said envelope, and a porous semi-fused silico-phosphate coatingon said quartz envelope, said light source is a compact incandescentfilament and said coating is on a portion of the circumference of saidenvelope in register with said filament and covers an area substantiallygreater than the area correspending to the product of the length-andoutside diameter of said filament, whereby the effective size of thelight source is increased by reflection from the coating.

2. The lamp of claim 1, in which the coating is confined to the portionof the envelope adjacent the filament.

3. The lamp of claim 2, in which the compact incan-- descent filament isa single coiled-coil filamentvs'ubstan tially axial of the envelope.

4. The combination of claim 1 in which the silicophosphate coatingcontains a fluoride to improve the References Cited UNITED STATESPATENTS 2,144,438 1/1939 Birdseye 313-113 2,568,459 9/1951 Noel 3132202,806,968 1/ 1957 Thorington et a1. 313-116 X 2,877,139 3/1959 Hyde3l3l16 X 3,325,662 6/1967 Cook 313-113 X JAMES W. LAWRENCE, PrimaryExaminer D. OREILLY, Assistant Examiner U.S. Cl. X.R.

